The present invention is directed to apparatus and method to measure the characteristics of the motion of objects undergoing substantially periodic motion using image analysis. Though developed to characterize the motions of microelectromechanical systems ("MEMS"), and, in particular, to test MEMS resonators, the present invention can be applied to any oscillating system.
MEMS resonators represent a fundamental type within a class of relatively new technologies developed by micromachining silicon and other materials. MEMS devices are expected to become part of future generations of communication, navigation, and information handling systems because of their simplicity, small size, and low power requirements. Methods of fabricating standard integrated circuits are compatible with making MEMS.
MEMS resonators show special promise as oscillators, filters, and mixers at radio frequencies. They can also function as accelerometers and gyroscopes in location-finding devices. Unlike conventional microelectronic devices, MEMS resonators have moving parts. Thus characterizing their operation requires analyzing images recorded while they operate.
Designing, fabricating, and testing MEMS devices require tools to verify that their dimensions, motions, and electrical signals substantially meet the designer's intent. Tools that accomplish these tasks automatically at the wafer level are especially desirable.
Tools that characterize electrical behavior are readily available from VLSI technology. The challenge is to combine both electrical and optical testing to simultaneously examine the motions that are the distinguishing characteristic of MEMS (and other periodic) devices. Further, a MEMS resonator may or may not have sensing means built in to enable electrical measurements that characterize the operation of the DUT. Even if a MEMS resonator has such means, the electrical measurement may require sophisticated equipment or circuits. Optical measurements are therefore preferred.
The most common optical measurements of DUT motion in the prior art require manually controlled test equipment, measuring the magnitude of DUT motion from visual observations under a microscope. There have been reports of more sophisticated techniques: measurements with a laser vibrometer or analysis of a series of time-resolved images produced by strobed illumination. But these techniques have limitations. Laser vibrometry is a spot method applicable mostly to measurements in the Z-plane (that is, perpendicular to the DUT's surface). Stroboscopy, and a derivative that combines interferometry with a strobed illumination source, measure motions in three dimensions using registration algorithms with six degrees of freedom. However, these stroboscopic techniques are limited to DUT motions that fall below an upper frequency limit set by how fast the source of illumination can be strobed, i.e., turned on and off.
Thus there exists a need for apparatus and methods of measuring DUT motions that is precise, sophisticated, and not subject to the limitations of the prior art.